people:fer:401ws:fall2018:previous

Math 401 - 01 Previous Homework (Fall 2018)

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Problem Set 10 (complete) Due: 11/06/2018. Board presentation: 11/20/2018

Let $G$ be a group, and $H,K\leq G$ .
1. Prove that if $HK=KH$ , then $HK\leq G$ .
2. Prove that if $H\leq N_G(K)$ , then $HK\leq G$ .
Let $G$ be a group, $H\leq G$ , and $C=\{gHg^{-1}|g\in G\}$ the set of all conjugates of $H$ in $G$ . Prove that: $|C|=[G:N_G(H)].$
Let $G$ be a group of order $120$ . What are the possible values of $n_2$ , $n_3$ , and $n_5$ , i.e. the number of Sylow 2-subgroups, the number of Sylow 3-subgroups and the number of Sylow 5-subgroups?
How many groups of order $6727$ are there? Describe them. Justify your answers. Show all your work.

Problem Set 09 (complete) Due: 10/29/2018. Board presentation: 11/02/2018

Prove that, up to isomorphism, the direct product operation is commutative and associative.
Give an example of a group $G$ with two subgroups $H$ and $K$ such that $HK=G$ , $H\intersection K=1$ , $K\normaleq G$ , but $G$ is not isomorphic to the direct product $H\oplus K$ .
Let $G$ be a group, and $H,N\leq G$ . Prove that:
1. If $N\normaleq G$ , then $HN\leq G$ .
2. If both $H,N\normaleq G$ , then $HN\normaleq G$ .
Make a list of all abelian groups of order $2736$ . Express each of them using the “elementary divisors” form and the “invariant factors” form.

Problem Set 08 (complete) Due: 10/22/2018. Problem 4 may be resubmitted by 10/24/2018. Board presentation 10/29/2018

Let $n\in\N$ and $H\leq G$ such that $H$ is the only subgroup of $G$ of order $n$ . Show that $H\normaleq G$ . (Do not assume that $G$ is finite)
Let $n\in\N$ and $H\leq G$ such that $H$ is the only subgroup of $G$ of index $n$ . Show that $H\normaleq G$ . (Do not assume that $G$ is finite)
Combine the previous problem with problem 3 in Problem Set 6.
Let $p,q$ be primes such that $p < q$ and $p\not\mid (q-1)$ . Prove that, up to isomorphism, there is only one group of order $pq$ . (Hint: Use example 17, page 203, as a guide. No use this example, you may use the extra assumption that $(p-1)\not\mid (q-1)$ , or equivalently that $(p-1)\not\mid (pq-1)$ .)

Problem Set 07 (complete) Due: 10/15/2018. Board presentation 10/29/2018

Prove Thm. 6.2.3, Thm. 6.3.2, Thm. 10.2.3. Combine all three proofs into one.
Chapter 10, problems 8, 10.

Problem Set 06 (complete) Due: 10/08/2018. Board presentation 10/10/2018

Chapter 7, problem 8.
Chapter 7, problem 22.
Let $G$ be a finite group, and $p$ the smallest prime divisor of $|G|$ . If $p^2\not\mid|G|$ , then $G$ has at most one subgroup of index $p$ . (Hint:Look at Example 6 on page 144)
Chapter 7, problem 12. Generalize.
Chapter 7, problem 48.

Problem Set 05 (complete) Due: 09/24/2018. Board presentation 09/28/2018

Chapter 5, problems 6, 8. For all of them find the order and the parity.
Chapter 5, problem 10. What is the largest order of an element of $S_8$ . Explain.
Chapter 5, problems 23, 24.
Chapter 5, problem 48.
Chapter 5, problem 50. Is $D_5$ a subgroup of $A_5$ ? Explain.

Problem Set 04 (complete) Due: 09/17/2018. Board presentation: 09/24/2018

Chapter 4, problem 74.
Chapter 5, problem 2.a.
Chapter 5, problem 4.
Consider $\alpha\in S_8$ given in disjoint cycle form by $\alpha=(1\ 4\ 5)(3\ 7)$ . Write $\alpha$ in array form.

Problem Set 03 (complete) Due: 09/12/2018. Board presentation: 09/17/2018

Let $G=\pbr{a}$ be an infinite cyclic group, and $k_1,k_2\in\Z$ . Prove that $\pbr{a^{k_1}}\leq\pbr{a^{k_2}} \textrm{ iff } k_2\mid k_1.$
Let $G=\pbr{a}$ be a cyclic group of order $60$ .
1. How many subgroups does $G$ have?
2. Which of them are cyclic?
3. List a generator for each of the cyclic subgroups of $G$ .
4. Draw the subgroup lattice of $G$ .
Prove that a finite group of prime order must be cyclic.
Chap. 4, problem 38, 62.
Chap. 4, problem 50.

Problem Set 02 (complete) Due: 09/04/2018. Board presentation: 09/12/2018

Chap. 3, problems 4, 13, 20, 64
Chap. 3, problems 6, 50
Let $G$ be a group in which every non-identity element has order 2. Prove that $G$ must be Abelian.
Chap. 3, problem 34. what can you say about the union of two subgroups?

Problem Set 01 (complete) Due: 08/27/2018. Board presentation: 08/31/2018

Page 38, prob. 18. What happens if you replace each H with an A?
Page 39, prob. 22. Explain.
Page 54, prob. 4.
Page 56, prob. 22. Compare with problem 13 on page 38.

Additional problems to look at: 1.13 p.38, 1.14 p.38, 2.5 p.54,

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